In conventional practice, supermarkets and convenient stores are equipped with display cases, which may be open or provided with doors, for presenting fresh food or beverages to customers, while maintaining the fresh food and beverages in a refrigerated environment. Typically, cold, moisture-bearing air is provided to the product display zone of each display case by passing air over the heat exchange surface of an evaporator coil disposed within the display case in a region separate from the product display zone so that the evaporator is out of customer view. A suitable refrigerant, such as for example R-404A refrigerant, is passed through the heat exchange tubes of the evaporator coil. As the refrigerant evaporates within the evaporator coil, heat is absorbed from the air passing over the evaporator so as to lower the temperature of the air.
A refrigeration system is installed in the supermarket and convenient store to provide refrigerant at the proper condition to the evaporator coils of the display cases within the facility. All refrigeration systems comprise at least the following components: a compressor, a condenser, at least one evaporator associated with a display case, a thermostatic expansion valve, and appropriate refrigerant lines connecting these devices in a closed circulation circuit. The thermostatic expansion valve is disposed in the refrigerant line upstream with respect to refrigerant flow of the inlet to the evaporator for expanding liquid refrigerant. The expansion valve functions to meter and expand the liquid refrigerant to a desired lower pressure, selected for the particular refrigerant, prior to entering the evaporator. As a result of this expansion, the temperature of the liquid refrigerant also drops significantly. The low pressure, low temperature liquid evaporates as it absorbs heat in passing through the evaporator tubes from the air passing over the surface of the evaporator. Typically, supermarket and grocery store refrigeration systems include multiple evaporators disposed in multiple display cases, an assembly of a plurality of compressors, termed a compressor rack, and one or more condensers.
Additionally, in certain refrigeration systems, an evaporator pressure regulator (EPR) valve is disposed in the refrigerant line at the outlet of the evaporator. The EPR valve functions to maintain the pressure within the evaporator above a predetermined pressure set point for the particular refrigerant being used. In refrigeration systems used to chill water, it is known to set the EPR valve so as to maintain the refrigerant within the evaporator above the freezing point of water. For example, in a water chilling refrigeration system using R-12 as refrigerant, the EPR valve may be set at a pressure set point of 32 psig (pounds per square inch, gage) which equates to a refrigerant temperature of 34 degrees F.
In conventional practice, evaporators in refrigerated food display systems generally operate with refrigerant temperatures below the frost point of water. Thus, frost will form on the evaporators during operation as moisture in the cooling air passing over the evaporator surface comes in contact with the evaporator surface. In medium-temperature refrigeration display cases, such as those commonly used for displaying produce, milk and other diary products, or meat, the refrigerated product must be maintained at a temperature typically in the range of 28 to 41 degrees F. depending upon the particular refrigerated product. In medium temperature produce display cases for example, conventional practice in the field of commercial refrigeration has been to pass the circulating cooling air over the tubes of an evaporator in which refrigerant passing through the tubes boils at about 21 degrees F. to maintain the cooling air temperature at about 31 or 32 degrees F. In medium temperature dairy product display cases for example, conventional practice in the commercial refrigeration field has been to pass the circulating cooling air over the tubes of an evaporator in which refrigerant passing through the tubes boils at about 21 degrees F. to maintain the cooling air temperature at about 28 or 29 degrees F. In medium temperature meat display cases for example, conventional practice in the commercial refrigeration field has been to pass the circulating cooling air over the tubes of an evaporator in which refrigerant boils at about 15 to 18 degrees F. to maintain the cooling air at a temperature of about 26 degrees F. At these refrigerant temperatures, the outside surface of the tube wall will be at a temperature below the frost point. As frost builds up on the evaporator surface, the performance of the evaporator deteriorates and the free flow of air through the evaporator becomes restricted and in extreme cases halted.
Conventional fin and tube heat exchanger coils used in forced air evaporators in the commercial refrigeration industry characteristically have a low fin density, typically having from 2 to 4 fins per inch. It has been conventional practice in the commercial refrigeration industry to use only heat exchangers of low fin density in evaporators for medium temperature and low temperature applications. This practice arises in anticipation of the buildup of frost of the surface of the evaporator heat exchanger and the desire to extend the period between required defrosting operations. As frost builds up, the effective flow space for air to pass between neighboring fins becomes progressively less and less until, in the extreme, the space is bridged with frost. As a consequence of frost buildup, heat exchanger performance decreases and the flow of adequately refrigerated air to the product display area decreases, thus necessitating activation of the defrost cycle.
Consequently, a conventional medium-temperature refrigerated food display system is customarily equipped with a defrost system that may be selectively or automatically operated to remove the frost formation from the evaporator surface, typically one to four times in a 24-hour period for up to one hundred and ten minutes each cycle. Conventional methods for defrosting evaporators on refrigerated food display systems include passing air over an electric heating element and thence over the evaporator, passing ambient temperature store air over the evaporator, and passing hot refrigerant gas through the refrigerant lines to and through the evaporator. In accord with the latter method, commonly referred to as hot gas defrost, hot gaseous refrigerant from the compressor, typically at a temperature of about 75 to about 120 degrees F., passes through the evaporator, warming the evaporator heat exchanger coil. The latent heat given off by the condensing hot gaseous refrigerant melts the frost off the evaporator. The hot gaseous refrigerant condenses in the frosted evaporator and returns as condensed liquid to an accumulator, rather than directly to the compressor to prevent compressor flooding and possible damage.
Although effective to remove the frost and thereby reestablish proper air flow and evaporator operating conditions, defrosting the evaporator has drawbacks. As the cooling cycle must be interrupted during the defrost period, the product temperature rises during the defrost. Thus, product in the display merchandiser may be repeatedly subject to alternate periods of cooling and warming. Therefore, product temperature in a conventional medium-temperature supermarket merchandiser displaying food products may during the defrost cycle exceed the 41 degree F. temperature limit set by the United States Food and Drug Administration. Also, additional controls must be provided on the refrigeration system to properly sequence defrosting cycles, particularly in stores having multiple refrigerated merchandisers to ensure that all merchandisers are not in defrost cycles simultaneously. According, it would be desirable to operate a refrigerated merchandiser, in particular a medium temperature merchandiser, in a continuous essentially frost-free state without the necessity of employing a defrost cycle.
U.S. Pat. No. 3,577,744, Mercer, discloses a method of operating an open refrigerated display case in which the product zone remains frost-free and in which the evaporator coils remain ice-free. In the disclosed method, a small secondary evaporator unit is utilized to dry ambient air for storage under pressure. The cooled, dehydrated air is then metered into the primary cooling air flow and passed in intimate contact with the surfaces in the product zone. As the air in intimate contact with the surfaces is dehydrated, no frost is formed on the surfaces in the product zone.
U.S. Pat. No. 3,681,896, Velkoff, discloses controlling the formation of frost in heat exchangers, such as evaporators, by applying an electrostatic charge to the air-vapor stream and to water introduced into the stream. The charged water droplets induce coalescence of the water vapor in the air and the charged coalesced vapor and droplets collect on the surface of oppositely charged plates disposed upstream of the heat exchanger coils. Thus, the cooling air passing over the heat exchanger coils is relatively moisture-free and frost formation on the heat exchanger coils does not occur.
U.S. Pat. No. 4,272,969, Schwitzgebel, discloses a refrigerator for maintaining a high humidity, frost-free environment. An additional throttling element, for example a suction-pressure-regulating valve or a capillary pipe, is installed in the return line between the evaporator outlet and the compressor for throttling the flow to maintain the evaporator surface above 0 degrees Centigrade. Additionally, the evaporator surface is sized far bigger than the evaporator surface used in conventional refrigerators of the same refrigerated volume, preferably twice the size of a conventional evaporator, and possibly ten times the size of a conventional evaporator.